//
// execution/relationship.hpp
// ~~~~~~~~~~~~~~~~~~~~~~~~~~
//
// Copyright (c) 2003-2022 Christopher M. Kohlhoff (chris at kohlhoff dot com)
//
// Distributed under the Boost Software License, Version 1.0. (See accompanying
// file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
//

#ifndef ASIO_EXECUTION_RELATIONSHIP_HPP
#define ASIO_EXECUTION_RELATIONSHIP_HPP

#if defined(_MSC_VER) && (_MSC_VER >= 1200)
# pragma once
#endif // defined(_MSC_VER) && (_MSC_VER >= 1200)

#include "asio/detail/config.hpp"
#include "asio/detail/type_traits.hpp"
#include "asio/execution/executor.hpp"
#include "asio/execution/scheduler.hpp"
#include "asio/execution/sender.hpp"
#include "asio/is_applicable_property.hpp"
#include "asio/query.hpp"
#include "asio/traits/query_free.hpp"
#include "asio/traits/query_member.hpp"
#include "asio/traits/query_static_constexpr_member.hpp"
#include "asio/traits/static_query.hpp"
#include "asio/traits/static_require.hpp"

#include "asio/detail/push_options.hpp"

namespace asio {

#if defined(GENERATING_DOCUMENTATION)

    namespace execution {

    /// A property to describe whether submitted tasks represent continuations of
    /// the calling context.
    struct relationship_t
    {
      /// The relationship_t property applies to executors, senders, and schedulers.
      template <typename T>
      static constexpr bool is_applicable_property_v =
        is_executor_v<T> || is_sender_v<T> || is_scheduler_v<T>;

      /// The top-level relationship_t property cannot be required.
      static constexpr bool is_requirable = false;

      /// The top-level relationship_t property cannot be preferred.
      static constexpr bool is_preferable = false;

      /// The type returned by queries against an @c any_executor.
      typedef relationship_t polymorphic_query_result_type;

      /// A sub-property that indicates that the executor does not represent a
      /// continuation of the calling context.
      struct fork_t
      {
        /// The relationship_t::fork_t property applies to executors, senders, and
        /// schedulers.
        template <typename T>
        static constexpr bool is_applicable_property_v =
          is_executor_v<T> || is_sender_v<T> || is_scheduler_v<T>;

        /// The relationship_t::fork_t property can be required.
        static constexpr bool is_requirable = true;

        /// The relationship_t::fork_t property can be preferred.
        static constexpr bool is_preferable = true;

        /// The type returned by queries against an @c any_executor.
        typedef relationship_t polymorphic_query_result_type;

        /// Default constructor.
        constexpr fork_t();

        /// Get the value associated with a property object.
        /**
         * @returns fork_t();
         */
        static constexpr relationship_t value();
      };

      /// A sub-property that indicates that the executor represents a continuation
      /// of the calling context.
      struct continuation_t
      {
        /// The relationship_t::continuation_t property applies to executors,
        /// senders, and schedulers.
        template <typename T>
        static constexpr bool is_applicable_property_v =
          is_executor_v<T> || is_sender_v<T> || is_scheduler_v<T>;

        /// The relationship_t::continuation_t property can be required.
        static constexpr bool is_requirable = true;

        /// The relationship_t::continuation_t property can be preferred.
        static constexpr bool is_preferable = true;

        /// The type returned by queries against an @c any_executor.
        typedef relationship_t polymorphic_query_result_type;

        /// Default constructor.
        constexpr continuation_t();

        /// Get the value associated with a property object.
        /**
         * @returns continuation_t();
         */
        static constexpr relationship_t value();
      };

      /// A special value used for accessing the relationship_t::fork_t property.
      static constexpr fork_t fork;

      /// A special value used for accessing the relationship_t::continuation_t
      /// property.
      static constexpr continuation_t continuation;

      /// Default constructor.
      constexpr relationship_t();

      /// Construct from a sub-property value.
      constexpr relationship_t(fork_t);

      /// Construct from a sub-property value.
      constexpr relationship_t(continuation_t);

      /// Compare property values for equality.
      friend constexpr bool operator==(
          const relationship_t& a, const relationship_t& b) noexcept;

      /// Compare property values for inequality.
      friend constexpr bool operator!=(
          const relationship_t& a, const relationship_t& b) noexcept;
    };

    /// A special value used for accessing the relationship_t property.
    constexpr relationship_t relationship;

    } // namespace execution

#else // defined(GENERATING_DOCUMENTATION)

    namespace execution {
        namespace detail {
            namespace relationship {

                template<int I>
                struct fork_t;
                template<int I>
                struct continuation_t;

            } // namespace relationship

            template<int I = 0>
            struct relationship_t {
#if defined(ASIO_HAS_VARIABLE_TEMPLATES)
                template <typename T>
                ASIO_STATIC_CONSTEXPR(bool,
                  is_applicable_property_v = (
                    is_executor<T>::value
                      || conditional<
                          is_executor<T>::value,
                          false_type,
                          is_sender<T>
                        >::type::value
                      || conditional<
                          is_executor<T>::value,
                          false_type,
                          is_scheduler<T>
                        >::type::value));
#endif // defined(ASIO_HAS_VARIABLE_TEMPLATES)

                ASIO_STATIC_CONSTEXPR(bool, is_requirable = false);

                ASIO_STATIC_CONSTEXPR(bool, is_preferable = false);

                typedef relationship_t polymorphic_query_result_type;

                typedef detail::relationship::fork_t<I> fork_t;
                typedef detail::relationship::continuation_t<I> continuation_t;

                ASIO_CONSTEXPR relationship_t()
                        : value_(-1) {
                }

                ASIO_CONSTEXPR relationship_t(fork_t)
                        : value_(0) {
                }

                ASIO_CONSTEXPR relationship_t(continuation_t)
                        : value_(1) {
                }

                template<typename T>
                struct proxy {
#if defined(ASIO_HAS_DEDUCED_QUERY_MEMBER_TRAIT)
                    struct type
                    {
                      template <typename P>
                      auto query(ASIO_MOVE_ARG(P) p) const
                        noexcept(
                          noexcept(
                            declval<typename conditional<true, T, P>::type>().query(
                              ASIO_MOVE_CAST(P)(p))
                          )
                        )
                        -> decltype(
                          declval<typename conditional<true, T, P>::type>().query(
                            ASIO_MOVE_CAST(P)(p))
                        );
                    };
#else // defined(ASIO_HAS_DEDUCED_QUERY_MEMBER_TRAIT)
                    typedef T type;
#endif // defined(ASIO_HAS_DEDUCED_QUERY_MEMBER_TRAIT)
                };

                template<typename T>
                struct static_proxy {
#if defined(ASIO_HAS_DEDUCED_QUERY_STATIC_CONSTEXPR_MEMBER_TRAIT)
                    struct type
                    {
                      template <typename P>
                      static constexpr auto query(ASIO_MOVE_ARG(P) p)
                        noexcept(
                          noexcept(
                            conditional<true, T, P>::type::query(ASIO_MOVE_CAST(P)(p))
                          )
                        )
                        -> decltype(
                          conditional<true, T, P>::type::query(ASIO_MOVE_CAST(P)(p))
                        )
                      {
                        return T::query(ASIO_MOVE_CAST(P)(p));
                      }
                    };
#else // defined(ASIO_HAS_DEDUCED_QUERY_STATIC_CONSTEXPR_MEMBER_TRAIT)
                    typedef T type;
#endif // defined(ASIO_HAS_DEDUCED_QUERY_STATIC_CONSTEXPR_MEMBER_TRAIT)
                };

                template<typename T>
                struct query_member :
                        traits::query_member<typename proxy<T>::type, relationship_t> {
                };

                template<typename T>
                struct query_static_constexpr_member :
                        traits::query_static_constexpr_member<
                                typename static_proxy<T>::type, relationship_t> {
                };

#if defined(ASIO_HAS_DEDUCED_STATIC_QUERY_TRAIT) \
 && defined(ASIO_HAS_SFINAE_VARIABLE_TEMPLATES)
                template <typename T>
                static ASIO_CONSTEXPR
                typename query_static_constexpr_member<T>::result_type
                static_query()
                  ASIO_NOEXCEPT_IF((
                    query_static_constexpr_member<T>::is_noexcept))
                {
                  return query_static_constexpr_member<T>::value();
                }

                template <typename T>
                static ASIO_CONSTEXPR
                typename traits::static_query<T, fork_t>::result_type
                static_query(
                    typename enable_if<
                      !query_static_constexpr_member<T>::is_valid
                    >::type* = 0,
                    typename enable_if<
                      !query_member<T>::is_valid
                    >::type* = 0,
                    typename enable_if<
                      traits::static_query<T, fork_t>::is_valid
                    >::type* = 0) ASIO_NOEXCEPT
                {
                  return traits::static_query<T, fork_t>::value();
                }

                template <typename T>
                static ASIO_CONSTEXPR
                typename traits::static_query<T, continuation_t>::result_type
                static_query(
                    typename enable_if<
                      !query_static_constexpr_member<T>::is_valid
                    >::type* = 0,
                    typename enable_if<
                      !query_member<T>::is_valid
                    >::type* = 0,
                    typename enable_if<
                      !traits::static_query<T, fork_t>::is_valid
                    >::type* = 0,
                    typename enable_if<
                      traits::static_query<T, continuation_t>::is_valid
                    >::type* = 0) ASIO_NOEXCEPT
                {
                  return traits::static_query<T, continuation_t>::value();
                }

                template <typename E,
                    typename T = decltype(relationship_t::static_query<E>())>
                static ASIO_CONSTEXPR const T static_query_v
                  = relationship_t::static_query<E>();
#endif // defined(ASIO_HAS_DEDUCED_STATIC_QUERY_TRAIT)
                //   && defined(ASIO_HAS_SFINAE_VARIABLE_TEMPLATES)

                friend ASIO_CONSTEXPR bool operator==(
                        const relationship_t &a, const relationship_t &b) {
                    return a.value_ == b.value_;
                }

                friend ASIO_CONSTEXPR bool operator!=(
                        const relationship_t &a, const relationship_t &b) {
                    return a.value_ != b.value_;
                }

                struct convertible_from_relationship_t {
                    ASIO_CONSTEXPR convertible_from_relationship_t(relationship_t) {
                    }
                };

                template<typename Executor>
                friend ASIO_CONSTEXPR relationship_t

                query(
                        const Executor &ex, convertible_from_relationship_t,
                        typename enable_if<
                                can_query<const Executor &, fork_t>::value
                        >::type * = 0)

#if !defined(__clang__) // Clang crashes if noexcept is used here.
#if defined(ASIO_MSVC) // Visual C++ wants the type to be qualified.
                ASIO_NOEXCEPT_IF((
                  is_nothrow_query<const Executor&, relationship_t<>::fork_t>::value))
#else // defined(ASIO_MSVC)
                ASIO_NOEXCEPT_IF((
                        is_nothrow_query<const Executor &, fork_t>::value))
#endif // defined(ASIO_MSVC)
#endif // !defined(__clang__)
                        {
                                return asio::query(ex, fork_t());
                        }

                template<typename Executor>
                friend ASIO_CONSTEXPR relationship_t

                query(
                        const Executor &ex, convertible_from_relationship_t,
                        typename enable_if<
                                !can_query<const Executor &, fork_t>::value
                        >::type * = 0,
                        typename enable_if<
                                can_query<const Executor &, continuation_t>::value
                        >::type * = 0)

#if !defined(__clang__) // Clang crashes if noexcept is used here.
#if defined(ASIO_MSVC) // Visual C++ wants the type to be qualified.
                ASIO_NOEXCEPT_IF((
                  is_nothrow_query<const Executor&,
                    relationship_t<>::continuation_t>::value))
#else // defined(ASIO_MSVC)
                ASIO_NOEXCEPT_IF((
                        is_nothrow_query<const Executor &, continuation_t>::value))
#endif // defined(ASIO_MSVC)
#endif // !defined(__clang__)
                        {
                                return asio::query(ex, continuation_t());
                        }

                ASIO_STATIC_CONSTEXPR_DEFAULT_INIT(fork_t, fork
                );
                ASIO_STATIC_CONSTEXPR_DEFAULT_INIT(continuation_t, continuation
                );

#if !defined(ASIO_HAS_CONSTEXPR)
                static const relationship_t instance;
#endif // !defined(ASIO_HAS_CONSTEXPR)

            private:
                int value_;
            };

#if defined(ASIO_HAS_DEDUCED_STATIC_QUERY_TRAIT) \
 && defined(ASIO_HAS_SFINAE_VARIABLE_TEMPLATES)
            template <int I> template <typename E, typename T>
            const T relationship_t<I>::static_query_v;
#endif // defined(ASIO_HAS_DEDUCED_STATIC_QUERY_TRAIT)
            //   && defined(ASIO_HAS_SFINAE_VARIABLE_TEMPLATES)

#if !defined(ASIO_HAS_CONSTEXPR)
            template<int I>
            const relationship_t<I> relationship_t<I>::instance;
#endif

            template<int I>
            const typename relationship_t<I>::fork_t
                    relationship_t<I>::fork;

            template<int I>
            const typename relationship_t<I>::continuation_t
                    relationship_t<I>::continuation;

            namespace relationship {

                template<int I = 0>
                struct fork_t {
#if defined(ASIO_HAS_VARIABLE_TEMPLATES)
                    template <typename T>
                    ASIO_STATIC_CONSTEXPR(bool,
                      is_applicable_property_v = (
                        is_executor<T>::value
                          || conditional<
                              is_executor<T>::value,
                              false_type,
                              is_sender<T>
                            >::type::value
                          || conditional<
                              is_executor<T>::value,
                              false_type,
                              is_scheduler<T>
                            >::type::value));
#endif // defined(ASIO_HAS_VARIABLE_TEMPLATES)

                    ASIO_STATIC_CONSTEXPR(bool, is_requirable = true);

                    ASIO_STATIC_CONSTEXPR(bool, is_preferable = true);

                    typedef relationship_t<I> polymorphic_query_result_type;

                    ASIO_CONSTEXPR fork_t() {
                    }

                    template<typename T>
                    struct query_member :
                            traits::query_member<
                                    typename relationship_t<I>::template proxy<T>::type, fork_t> {
                    };

                    template<typename T>
                    struct query_static_constexpr_member :
                            traits::query_static_constexpr_member<
                                    typename relationship_t<I>::template static_proxy<T>::type, fork_t> {
                    };

#if defined(ASIO_HAS_DEDUCED_STATIC_QUERY_TRAIT) \
 && defined(ASIO_HAS_SFINAE_VARIABLE_TEMPLATES)
                    template <typename T>
                    static ASIO_CONSTEXPR
                    typename query_static_constexpr_member<T>::result_type
                    static_query()
                      ASIO_NOEXCEPT_IF((
                        query_static_constexpr_member<T>::is_noexcept))
                    {
                      return query_static_constexpr_member<T>::value();
                    }

                    template <typename T>
                    static ASIO_CONSTEXPR fork_t static_query(
                        typename enable_if<
                          !query_static_constexpr_member<T>::is_valid
                        >::type* = 0,
                        typename enable_if<
                          !query_member<T>::is_valid
                        >::type* = 0,
                        typename enable_if<
                          !traits::query_free<T, fork_t>::is_valid
                        >::type* = 0,
                        typename enable_if<
                          !can_query<T, continuation_t<I> >::value
                        >::type* = 0) ASIO_NOEXCEPT
                    {
                      return fork_t();
                    }

                    template <typename E, typename T = decltype(fork_t::static_query<E>())>
                    static ASIO_CONSTEXPR const T static_query_v
                      = fork_t::static_query<E>();
#endif // defined(ASIO_HAS_DEDUCED_STATIC_QUERY_TRAIT)
                    //   && defined(ASIO_HAS_SFINAE_VARIABLE_TEMPLATES)

                    static ASIO_CONSTEXPR relationship_t<I>

                    value() {
                        return fork_t();
                    }

                    friend ASIO_CONSTEXPR bool operator==(
                            const fork_t &, const fork_t &) {
                        return true;
                    }

                    friend ASIO_CONSTEXPR bool operator!=(
                            const fork_t &, const fork_t &) {
                        return false;
                    }
                };

#if defined(ASIO_HAS_DEDUCED_STATIC_QUERY_TRAIT) \
 && defined(ASIO_HAS_SFINAE_VARIABLE_TEMPLATES)
                template <int I> template <typename E, typename T>
                const T fork_t<I>::static_query_v;
#endif // defined(ASIO_HAS_DEDUCED_STATIC_QUERY_TRAIT)
                //   && defined(ASIO_HAS_SFINAE_VARIABLE_TEMPLATES)

                template<int I = 0>
                struct continuation_t {
#if defined(ASIO_HAS_VARIABLE_TEMPLATES)
                    template <typename T>
                    ASIO_STATIC_CONSTEXPR(bool,
                      is_applicable_property_v = (
                        is_executor<T>::value
                          || conditional<
                              is_executor<T>::value,
                              false_type,
                              is_sender<T>
                            >::type::value
                          || conditional<
                              is_executor<T>::value,
                              false_type,
                              is_scheduler<T>
                            >::type::value));
#endif // defined(ASIO_HAS_VARIABLE_TEMPLATES)

                    ASIO_STATIC_CONSTEXPR(bool, is_requirable = true);

                    ASIO_STATIC_CONSTEXPR(bool, is_preferable = true);

                    typedef relationship_t<I> polymorphic_query_result_type;

                    ASIO_CONSTEXPR continuation_t() {
                    }

                    template<typename T>
                    struct query_member :
                            traits::query_member<
                                    typename relationship_t<I>::template proxy<T>::type, continuation_t> {
                    };

                    template<typename T>
                    struct query_static_constexpr_member :
                            traits::query_static_constexpr_member<
                                    typename relationship_t<I>::template static_proxy<T>::type,
                                    continuation_t> {
                    };

#if defined(ASIO_HAS_DEDUCED_STATIC_QUERY_TRAIT) \
 && defined(ASIO_HAS_SFINAE_VARIABLE_TEMPLATES)
                    template <typename T>
                    static ASIO_CONSTEXPR
                    typename query_static_constexpr_member<T>::result_type
                    static_query()
                      ASIO_NOEXCEPT_IF((
                        query_static_constexpr_member<T>::is_noexcept))
                    {
                      return query_static_constexpr_member<T>::value();
                    }

                    template <typename E,
                        typename T = decltype(continuation_t::static_query<E>())>
                    static ASIO_CONSTEXPR const T static_query_v
                      = continuation_t::static_query<E>();
#endif // defined(ASIO_HAS_DEDUCED_STATIC_QUERY_TRAIT)
                    //   && defined(ASIO_HAS_SFINAE_VARIABLE_TEMPLATES)

                    static ASIO_CONSTEXPR relationship_t<I>

                    value() {
                        return continuation_t();
                    }

                    friend ASIO_CONSTEXPR bool operator==(
                            const continuation_t &, const continuation_t &) {
                        return true;
                    }

                    friend ASIO_CONSTEXPR bool operator!=(
                            const continuation_t &, const continuation_t &) {
                        return false;
                    }
                };

#if defined(ASIO_HAS_DEDUCED_STATIC_QUERY_TRAIT) \
 && defined(ASIO_HAS_SFINAE_VARIABLE_TEMPLATES)
                template <int I> template <typename E, typename T>
                const T continuation_t<I>::static_query_v;
#endif // defined(ASIO_HAS_DEDUCED_STATIC_QUERY_TRAIT)
                //   && defined(ASIO_HAS_SFINAE_VARIABLE_TEMPLATES)

            } // namespace relationship
        } // namespace detail

        typedef detail::relationship_t<> relationship_t;

#if defined(ASIO_HAS_CONSTEXPR) || defined(GENERATING_DOCUMENTATION)
        constexpr relationship_t relationship;
#else // defined(ASIO_HAS_CONSTEXPR) || defined(GENERATING_DOCUMENTATION)
        namespace {
            static const relationship_t &
                    relationship = relationship_t::instance;
        }
#endif

    } // namespace execution

#if !defined(ASIO_HAS_VARIABLE_TEMPLATES)

    template<typename T>
    struct is_applicable_property<T, execution::relationship_t>
            : integral_constant<bool,
                    execution::is_executor<T>::value
                    || conditional<
                            execution::is_executor<T>::value,
                            false_type,
                            execution::is_sender<T>
                    >::type::value
                    || conditional<
                            execution::is_executor<T>::value,
                            false_type,
                            execution::is_scheduler<T>
                    >::type::value> {
    };

    template<typename T>
    struct is_applicable_property<T, execution::relationship_t::fork_t>
            : integral_constant<bool,
                    execution::is_executor<T>::value
                    || conditional<
                            execution::is_executor<T>::value,
                            false_type,
                            execution::is_sender<T>
                    >::type::value
                    || conditional<
                            execution::is_executor<T>::value,
                            false_type,
                            execution::is_scheduler<T>
                    >::type::value> {
    };

    template<typename T>
    struct is_applicable_property<T, execution::relationship_t::continuation_t>
            : integral_constant<bool,
                    execution::is_executor<T>::value
                    || conditional<
                            execution::is_executor<T>::value,
                            false_type,
                            execution::is_sender<T>
                    >::type::value
                    || conditional<
                            execution::is_executor<T>::value,
                            false_type,
                            execution::is_scheduler<T>
                    >::type::value> {
    };

#endif // !defined(ASIO_HAS_VARIABLE_TEMPLATES)

    namespace traits {

#if !defined(ASIO_HAS_DEDUCED_QUERY_FREE_TRAIT)

        template<typename T>
        struct query_free_default<T, execution::relationship_t,
                typename enable_if<
                        can_query<T, execution::relationship_t::fork_t>::value
                >::type> {
            ASIO_STATIC_CONSTEXPR(bool, is_valid = true);

            ASIO_STATIC_CONSTEXPR(bool, is_noexcept =
            (is_nothrow_query<T, execution::relationship_t::fork_t>::value));

            typedef execution::relationship_t result_type;
        };

        template<typename T>
        struct query_free_default<T, execution::relationship_t,
                typename enable_if<
                        !can_query<T, execution::relationship_t::fork_t>::value
                        && can_query<T, execution::relationship_t::continuation_t>::value
                >::type> {
            ASIO_STATIC_CONSTEXPR(bool, is_valid = true);

            ASIO_STATIC_CONSTEXPR(bool, is_noexcept =
            (is_nothrow_query<T, execution::relationship_t::continuation_t>::value));

            typedef execution::relationship_t result_type;
        };

#endif // !defined(ASIO_HAS_DEDUCED_QUERY_FREE_TRAIT)

#if !defined(ASIO_HAS_DEDUCED_STATIC_QUERY_TRAIT) \
 || !defined(ASIO_HAS_SFINAE_VARIABLE_TEMPLATES)

        template<typename T>
        struct static_query<T, execution::relationship_t,
                typename enable_if<
                        execution::detail::relationship_t<0>::
                        query_static_constexpr_member<T>::is_valid
                >::type> {
            ASIO_STATIC_CONSTEXPR(bool, is_valid = true);

            ASIO_STATIC_CONSTEXPR(bool, is_noexcept = true);

            typedef typename execution::detail::relationship_t<0>::
            query_static_constexpr_member<T>::result_type result_type;

            static ASIO_CONSTEXPR result_type

            value() {
                return execution::detail::relationship_t<0>::
                query_static_constexpr_member<T>::value();
            }
        };

        template<typename T>
        struct static_query<T, execution::relationship_t,
                typename enable_if<
                        !execution::detail::relationship_t<0>::
                        query_static_constexpr_member<T>::is_valid
                        && !execution::detail::relationship_t<0>::
                        query_member<T>::is_valid
                        && traits::static_query<T,
                                execution::relationship_t::fork_t>::is_valid
                >::type> {
            ASIO_STATIC_CONSTEXPR(bool, is_valid = true);

            ASIO_STATIC_CONSTEXPR(bool, is_noexcept = true);

            typedef typename traits::static_query<T,
                    execution::relationship_t::fork_t>::result_type result_type;

            static ASIO_CONSTEXPR result_type

            value() {
                return traits::static_query<T,
                        execution::relationship_t::fork_t>::value();
            }
        };

        template<typename T>
        struct static_query<T, execution::relationship_t,
                typename enable_if<
                        !execution::detail::relationship_t<0>::
                        query_static_constexpr_member<T>::is_valid
                        && !execution::detail::relationship_t<0>::
                        query_member<T>::is_valid
                        && !traits::static_query<T,
                                execution::relationship_t::fork_t>::is_valid
                        && traits::static_query<T,
                                execution::relationship_t::continuation_t>::is_valid
                >::type> {
            ASIO_STATIC_CONSTEXPR(bool, is_valid = true);

            ASIO_STATIC_CONSTEXPR(bool, is_noexcept = true);

            typedef typename traits::static_query<T,
                    execution::relationship_t::continuation_t>::result_type result_type;

            static ASIO_CONSTEXPR result_type

            value() {
                return traits::static_query<T,
                        execution::relationship_t::continuation_t>::value();
            }
        };

        template<typename T>
        struct static_query<T, execution::relationship_t::fork_t,
                typename enable_if<
                        execution::detail::relationship::fork_t<0>::
                        query_static_constexpr_member<T>::is_valid
                >::type> {
            ASIO_STATIC_CONSTEXPR(bool, is_valid = true);

            ASIO_STATIC_CONSTEXPR(bool, is_noexcept = true);

            typedef typename execution::detail::relationship::fork_t<0>::
            query_static_constexpr_member<T>::result_type result_type;

            static ASIO_CONSTEXPR result_type

            value() {
                return execution::detail::relationship::fork_t<0>::
                query_static_constexpr_member<T>::value();
            }
        };

        template<typename T>
        struct static_query<T, execution::relationship_t::fork_t,
                typename enable_if<
                        !execution::detail::relationship::fork_t<0>::
                        query_static_constexpr_member<T>::is_valid
                        && !execution::detail::relationship::fork_t<0>::
                        query_member<T>::is_valid
                        && !traits::query_free<T,
                                execution::relationship_t::fork_t>::is_valid
                        && !can_query<T, execution::relationship_t::continuation_t>::value
                >::type> {
            ASIO_STATIC_CONSTEXPR(bool, is_valid = true);

            ASIO_STATIC_CONSTEXPR(bool, is_noexcept = true);

            typedef execution::relationship_t::fork_t result_type;

            static ASIO_CONSTEXPR result_type

            value() {
                return result_type();
            }
        };

        template<typename T>
        struct static_query<T, execution::relationship_t::continuation_t,
                typename enable_if<
                        execution::detail::relationship::continuation_t<0>::
                        query_static_constexpr_member<T>::is_valid
                >::type> {
            ASIO_STATIC_CONSTEXPR(bool, is_valid = true);

            ASIO_STATIC_CONSTEXPR(bool, is_noexcept = true);

            typedef typename execution::detail::relationship::continuation_t<0>::
            query_static_constexpr_member<T>::result_type result_type;

            static ASIO_CONSTEXPR result_type

            value() {
                return execution::detail::relationship::continuation_t<0>::
                query_static_constexpr_member<T>::value();
            }
        };

#endif // !defined(ASIO_HAS_DEDUCED_STATIC_QUERY_TRAIT)
        //   || !defined(ASIO_HAS_SFINAE_VARIABLE_TEMPLATES)

#if !defined(ASIO_HAS_DEDUCED_STATIC_REQUIRE_TRAIT)

        template<typename T>
        struct static_require<T, execution::relationship_t::fork_t,
                typename enable_if<
                        static_query<T, execution::relationship_t::fork_t>::is_valid
                >::type> {
            ASIO_STATIC_CONSTEXPR(bool, is_valid =
            (is_same<typename static_query<T,
                    execution::relationship_t::fork_t>::result_type,
                    execution::relationship_t::fork_t>::value));
        };

        template<typename T>
        struct static_require<T, execution::relationship_t::continuation_t,
                typename enable_if<
                        static_query<T, execution::relationship_t::continuation_t>::is_valid
                >::type> {
            ASIO_STATIC_CONSTEXPR(bool, is_valid =
            (is_same<typename static_query<T,
                    execution::relationship_t::continuation_t>::result_type,
                    execution::relationship_t::continuation_t>::value));
        };

#endif // !defined(ASIO_HAS_DEDUCED_STATIC_REQUIRE_TRAIT)

    } // namespace traits

#endif // defined(GENERATING_DOCUMENTATION)

} // namespace asio

#include "asio/detail/pop_options.hpp"

#endif // ASIO_EXECUTION_RELATIONSHIP_HPP
